Cyclic adenosine 3 ': 5 '-monophosphate-dependent protein kinase on the external surface of LS-174T human colon carcinoma cells

The analysis of purified plasma membranes and the surface of intact cells r evealed the presence of cyclic adenosine 3':5'-monophosphate-(cAMP) depende nt protein kinase (PKA) on the external surface of LS-174T human colon carc inoma cells. Photoaffinity labeling of intact cells at confluence with 8-az ido[P-32]cAMP identified the cAMP-binding proteins on the surface. Immunopr ecipitation identified the photoaffinity-labeled cAMP-binding proteins as t he RII alpha regulatory. subunit of PKA. During the logarithmic stage of gr owth, both the RI alpha and RII alpha subunits of PKA were localized on the cell surface. Intact LS-174T cells catalyzed the phosphorylation of Kempti de in a cAMP-dependent manner, upon substitution of cAMP in the medium with 8-chloroadenosine, which did not compete with cAMP for the binding on inta ct cells, the ecto-PKA was no longer activated. The specific inhibitory pro tein for PKA, PKI, abolished the stimulation of phosphorylation by cAMP. Fo rskolin, which elevates intracellular levels of cAMP, activated ecto-PKA. M oreover, probenecid, which blocks the export of cAMP, inhibited the forskol in-mediated activation of ecto-PKA. These results demonstrate that LS-174T colon carcinoma cells possess an ecto-PKA on the external surface. This ect o-PICA is similar, if not identical, to the soluble intracellular PKA.